At present, fuel cell is watched with interest as a candidate for the power generation system friendly to the terrestrial environment substituting for thermal power generation. The fuel cell is not the kind of the so-called dry cell or storage cell used by storing electricity. While water generates hydrogen and oxygen when electrically hydrolyzed, the fuel cell utilizes a principle reverse thereto. In other words, the fuel cell is a new power generation system directly converting chemical energy to electric energy by electrochemical reaction of hydrogen and oxygen through a catalyst or the like.
The fuel cell is not restricted by the Carnot cycle since the same is a power generation system directly converting chemical energy to electric energy, and theoretically has remarkably superior generation efficiency as compared with thermal power generation since the same causes neither heat transfer loss nor mechanical loss. However, even the fuel cell cannot entirely convert thermal energy obtained in combustion, i.e., change of enthalpy (denoted by ΔH) to electric energy but can merely convert change of Gibbs free energy (denoted by ΔG) to electric energy.
In practice, theoretically possible maximum efficiency (theoretical efficiency) of a fuel cell fueled by hydrogen is:H2 (gas)+1/2O2 (gas)→H2O (liquid)ΔH=−285.83 kJ/molΔG=−237.13 kJ/molHence,ΔG/ΔH×100=82.9%
Thus, the theoretical efficiency of the fuel cell exhibits a high value by far exceeding the theoretical efficiency of a heat engine shown by the Carnot cycle. Similarly, the theoretical efficiency exhibits a value exceeding 90% also when the fuel cell is fueled by methane or alcohol.
However, it is difficult to approach the theoretical efficiency in an actual fuel cell at the present stage. The reason for this is that various energy loss takes place in the fuel cell and a supplemental apparatus for the fuel cell and the energy is discharged from the fuel cell as thermal energy.
At present, loss in the fuel cell is the maximum loss in the power generation system formed by the fuel cell, and the power generation efficiency of the fuel cell can be remarkably improved by reducing such loss.
For the aforementioned reason, active research and development are performed in various fields, to develop various types of fuel cells such as phosphoric acid-, solid polymer-, molten carbonate- and solid electrolyte-type cells.
Among these fuel cells, the solid polymer electrolyte fuel cell (also abbreviated as PEFC) operating at a lower temperature as compared with other types of fuel cells has small constraint in the materials used such that the same can be prepared from a low-priced material such as plastic, carbon or stainless steel for readily reducing the cost, and is gathering interests. Further, the PEFC can be miniaturized as compared with other types of fuel cells and is suitable to a mobile power source or a small capacity power source.
The history of development of the PEFC was first started by General Electric, U.S.A., in the latter half of 1950s, and a fuel cell utilizing hydrogen/oxygen having output power of 1 kW was embarked on the spaceship Gemini in the first half of 1960s. While the polymer electrolyte membrane used at the beginning was a polystyrene membrane of which chemical durability was poor, the chemical durability of the PEFC has been remarkably improved by the fluoropolymer electrolyte membrane “Nafion (R)” developed by Du Pont, U.S.A. for fuel cells in the space development project of NASA, and development of the PEFC was stimulated. At present, application of the PEFC to automobiles or domestic use thereof is mainly studied rather than conventional space or military use.
The polymer electrolyte membrane forming the heart of the PEFC serves as a kind of ion-exchange membrane, and must have excellent ion conductivity, physical strength, gas barrier properties, chemical stability, electrochemical stability and thermal stability. Therefore, a perfluorocarbonsulfonic acid membrane represented by “Nafion (R)” by Du Pont, U.S.A. has mainly been used as a polymer electrolyte membrane usable over a long period. In general, the perfluorocarbonsulfonic acid membrane has fluorine atoms in the main chain and side chains, and sulfonic acid groups in the side chains to which protons can be added.
When the general perfluorocarbonsulfonic acid membrane is operated under a condition exceeding 100° C., however, the water content of the membrane is abruptly reduced and the membrane is remarkably softened. In a direct methanol fuel cell, therefore, performance is so reduced due to methanol crossover in the membrane that the fuel cell cannot exhibit sufficient performance. In a fuel cell fueled by hydrogen and operated under a condition around 80° C., the high cost of perfluorocarbonsulfonic acid membrane also hinders practicalization of the fuel cell. Moreover, available perfluorocarbonsulfonic acid membrane limited in thickness and ion exchange capacity and designing of the fuel cell is also restricted.
In order to overcome such disadvantages, various types of electrolyte membranes prepared by introducing sulfonic acid groups into aromatic polymers are studied. For example, sulfonated polyaryl ethersulfone (Journal of Membrane Science, 83, 211 (1993)), sulfonated polyetherether ketone (Japanese Patent Laying-Open No. 6-93114), sulfonated polystyrene and the like can be listed. However, sulfonic acid groups introduced into aromatic rings based on polymer reaction readily cause desulfonation by acid or heat, and cannot be regarded as sufficient in durability to be used as an electrolyte membrane for a fUel cell.
An aromatic polyazole polymer such as polyimidazole is known as a polymer having high heat resistance and high durability, and sulfonic acid groups may conceivably be introduced into such a polymer for utilizing the same for the aforementioned object. As a compound having such a polymer structure, polybenzimidazole having sulfonic acids synthesized from 3,3′-diaminobenzidine and 3,5-dicarboxybenzensulfonic acid or 2,5-dicarboxybenzensulfonic acid (Uno et al., J. Polym. Sci., Polym. Chem., 15, 1309 (1977)) or synthesized with main components of 1,2,4,5-benzenetetramine and 2,5-dicarboxybenzensulfonic acid (U.S. Pat. No. 5,312,895) has been reported.
In these reports, although solubility and heat resistance of polybenzimidazole having sulfonic acid have been described, no attention has been directed to electrochemical properties of sulfonic acid groups such as application to a solid polymer electrolyte membrane. In particular, these compounds are inferior in compatibility of heat resistance, solvent resistance and mechanical properties and ion conductivity, and unsuitable for application to a solid polymer electrolyte membrane or the like.
Also as to that related to polybenzoxazole or polybenzthiazole having sulfonic acid groups, polymers synthesized from 2,5-diamino-1,4-benzenedithiol and 3,5-dicarboxybenzenesulfonic acid or 4,6-dicarboxy-1,3-benzenedisulfonic acid (J. Polym. Sci., Polym. Chem., 34, 481 (1996)), polymers synthesized from 2,5-diamino-1,4-benzenediol and 3,5-dicarboxybenzenesulfonic acid (Japanese Patent Laying-Open No. 10-158213), a compound prepared by sulfonation of a compound synthesized from 2,5-diamino-1,4-benzenediol and terephthalic acid (Japanese Patent Laying-Open No. 4-353533) or a compound synthesized from 2,5-dicarboxysulfonic acid and various diaminediol or diaminedithiol (U.S. Pat. No. 5,492,996) has been reported.
However, none of these has noted sulfonic acid groups as functional groups for proton conductivity, and none has exhibited sufficient durability under a condition employed as a fuel cell. For example, while the technique disclosed in U.S. Pat. No. 5,492,996 is characterized in converting sulfonic acid groups to alkyl ammonium salt in order to derive alcohol solubility of the polymer, it is obvious that the same is unsuitable for use as the material for a fuel cell since alcohol solubility is a critical defect in application to the aforementioned fuel cell fueled by methanol or the like.
On the other hand, few aromatic polymers having phosphonic acid groups conceivably superior in heat resistance to sulfonic acid groups was reported in view of application to a solid polymer electrolyte. Polybenzoxazoles comprising of 4,4′-(2,2,2-trifluoro-1-(trifluoromethyl)ethylidene)bis(2-aminophenol) in which 5 to 50% of dicarboxylic acid was 3.5-dicarboxyphenyl phosphonic acid (U.S. Pat. No. 5,498,784) were reported as a rare example. These polymers showed excellent solubility and a possibility as a composite material, however, this polymer has not been taken into consideration as a solid polyelectrolyte for a fuel cell. In practice, it is obvious that alcohol solubility of these polymers is unsuitable for use as a solid polyelectrolyte for a fuel cell fueled by methanol. It can be said that this polymer is unsuitable for a solid polyelectrolyte for a fuel cell also in the point that the same exhibits only low ion conductivity.
In addition, a phosphorus-containing polyamide copolymer such as 3,5-dicarboxyphenylphosphonic acid has been reported (Japanese Patent Laying-Open No. 11-286545), while only properties related to heat resistance have been investigated also in this polymer. Further, this polymer causes hydrolysis under an acidic condition used as a fuel cell, and cannot be used as an electrolyte membrane.
In general, ionic group-containing polyazole polymers are hard to mold and also hard to hold its form even if a molding is obtained. This is conceivably because only a polymer having a low degree of polymerization is obtained by introduction of ionic groups. Therefore, it has been difficult to obtain a polymer utilizable as a solid polymer electrolyte such as a proton-exchange membrane.